Method of providing brazed ferrous metal parts with aluminum coating



xresistant to high temperature oxidizing drapes atent 2,755,542 Patented July 24, 1956 Free METHOD OF PROVIDING BRAZED FERROUS NIETAL PARTS WITH ALUMINUM COATING No Drawing. Application May 14, 1949, Serial No. 93,407

6 Claims. (Cl. 29-527) This invention has to do with the formation of structures formed of a plurality of ferrous metal parts copper brazed together and treated to make the same resistant to oxidization at elevated temperatures.

Copper and copper base brazing alloys are excellent materials for joining steel or other ferrous metal parts into a strong assembly where the assembly is not subjected to high temperature oxidization. However, where the assembly is subjected to high temperature conditions the brazed joints fail by oxidization of the copper. As an illustrative example of a high temperature use, heat exchangers for gas turbines formed of steel parts copper brazed together may be mentioned.

The primary object of the present invention is to provide a strong copper-brazed structure that is highly resistant to oxidization at elevated temperatures.

Other objects and advantages of the invention will become more apparent as the description proceeds.

In accordance with the invention an assembly consisting of a plurality of steel or other ferrous metal parts is copper brazed in any known manner to bond the parts together to provide a strong unitary structure. Thereafter the copper brazed steel structure is dipped into molten aluminum or aluminum base alloy only long enough to produce surface alloying to a depth of about .0O02".0005" in both the steel and the copper. This the other hand, another type of copper brazed joint may have the copper substantially all shielded by the steel sections joined by the copper. In this case the molten aluminum will not have access for readily dissolving the copper so that assemblies of this kind can be dipped for longer periods, for example as long as 10 to 15 seconds.

In carrying out the preheating step exceptionally good results are obtained by immersing and heating the brazed assembly in a molten salt bath composed substantially as follows:

Percent Potassium chloride (KCl) 47 Sodium chloride (NaCl) Cryolite (NasAlFs) 12 Aluminum fluoride (AlFs) 6 The salt bath has a melting point of about 1180 F.

The exact composition of the salt bath is not sharply critical and the proportions of potassium chloride, sodium chloride, cryolite, and aluminum fluoride may be varied from the above. For example, the potassium chloride may range from about 37 to 57%, the sodium chloride from about 25 to 45%, the cryolite from about 8 to 20% and the aluminum fluoride from about 0.5 to 12%. The bath composition usually preferred is one that will become molten when heated to 1200 F. or somewhat lower, While in the foregoing examples the double salt NasAlFs (cryolite) is given it should be understood that an equivalent amount of this component may be supplied in the form of the single salts, sodium fluoride produces an aluminum coated assembly that is highly gases both in the steel and in the copper brazed joints.

The time the brazed assembly is held in the molten aluminum or aluminum base alloy bath is important. Copper is readily soluble in molten aluminum. Accordingly the copper brazed steel structure is permitted only a short time of contact with molten aluminum so as not to dissolve the copper joint excessively. It is essential also that the brazed assembly be at a temperature at least as high as that of the melting point of the aluminum or aluminum base alloy coating bath before it is removed therefrom. To satisfy these requirements and at the same time to produce an aluminum coating that completely covers and alloy bonds to both the steel and copper during the short time of contact with the coating bath the brazed assembly is preheated to a temperature at least as high as that of the melting point of the aluminum or aluminum base alloy bath in a fused salt bath capable of dissolving oxides of iron and aluminum. The preheated assembly is then immersed in the molten aluminum or aluminum base alloy coating bath for a time only long enough to provide surface alloying of the aluminum for a depth of approximately .0002- .0005" in both the steel and copper. A suitable ternperature for the molten aluminum or aluminum base alloy coating bath is about 1250l300 F. The physical character of the joint determines the allowable dipping time. A copper brazed joint may be such that a large portion of the copper is exposed at the surface. In such a case the dipping time has to be held to a minimum, for example, as little as 3 to 5 seconds. On

and aluminum fluoride. It is essential to provide an excess of AlFz over that of the cryolite ratio in order to obtain the desired results.

The temperature of the fused salt bath is maintained during operation at a temperature Within the approximate range of 1250 to 1600 F. In general a temperature of 1300 to 1400 F. is preferred.

The fused salt bath must be activated by aluminum in or in contact with the fused salt in order to provide effective fluxing action. The preheating salt bath and the aluminum coating bath may be combined in a single container with the layer of fused salt on top of the molten aluminum of sufiicient depth so that the assembly can be immersed in the fused salt to preheat the same prior to being lowered into the aluminum. The molten aluminum in contact with the salt activates the latter. Where the preheating salt bath is in a container separate from that for the aluminum coating metal it is essential to otherwise activate the fused salt. This may be done by adding aluminum or aluminum alloy to the salt. For example a bar or sheet of aluminum or aluminum alloy may be immersed in the molten salt where it soon melts and goes to the bottom of the salt bath. The preferred form of container for the fused salt is one that is lined with silicon carbide, i. e. Carborundum. An aluminized steel container may also be used to contain the fused salt and in this instance the aluminum at the inner surface of the container activates the fused salt.

After the brazed assembly has been immersed in the molten aluminum metal coating bath for a few seconds, it is removed from the bath. The coated assembly is a strongly bonded one that is highly resistant to oxidization at elevated temperatures on the order of l700 F. or even higher. Still further improvement is obtained when the brazed and coated assembly is heated to an approximate temperature of 1900 F. for a suflicient length of time for the aluminum to diffuse into the brazed assembly. A suitable time is 1 to 1 /2 hours. It is not necessary to employ a non-oxidizing atmosphere for the diffusing heat treatment.

37 to 57% KC] 25 to 45% NaCl 8 to 20% NasAlFe 0.5 to 12% AlFs said fused salt being activated by aluminum in contact therewith, and then immersing the heated assembly in a molten aluminum bath for ashort time to obtain an aluminum coating joined both to the ferrous metal and exposed copper brazing material of the brazed assembly by an alloy bond having a thickness of about .0002 to .0005".

2. The process as in claim 1 which includes the further step of reheating the coated assembly to a temperature of approximately 1900 F. for a time of approximately one to one and one-half hours.

3. A process as in claim 1 in which the time of immersion in the molten aluminum bath is within the approximate time range of 3 to 15 seconds.

4. A process as in claim 1 in which the salt bath consists substantially as follows: 47% KCl, 35% NaCl, 12% NasAlFs and AlFz 6%.

5. The process of forming a structure highly resistant to oxidization at elevated temperature which consists in copper brazing together a plurality of ferrous metal parts to provide an assembly of ferrous metal parts having exposed copper brazing material between the ferrous metal parts, preheating the assembly to a temperature at least as high as the melting point of aluminum in a salt bath consisting substantially as follows:

37 to 57% KCl 25 to 45% NaCl 8 to 20% NasAlFs 0.5 to 12% AlFs said fused salt being activated by aluminum in contact therewith, and then, while the brazed assembly is still substantially at said salt bath temperature immersing the heated assembly in a molten aluminum bath at a temperature of about 12501300 F. for a time of about 3 to 15 seconds to obtain an aluminum coating joined both to the ferrous metal and exposed copper brazing material of the brazed assembly by an alloy bond having a thickness of about .0002 to .0005".

6. The process as in claim 5 which includes the further step of reheating the coated assembly to a temperature of approximately 1900" F. for a time of approximately one to one and one-half hours.

References Cited in the file of this patent UNITED STATES PATENTS 723,717 Nicholson Mar. 24, 1903 838,211 Rennie Dec. 11, 1906 1,637,033 Basch July 26, 1927 1,914,269 Liban June 13, 1933 2,101,553 Mattson Dec. 7, 1937 2,135,652 Whitfield Nov. 8, 1938 2,166,510 Whitfield July 18, 1939 2,294,750 Harris Sept. 1, 1942 2,315,725 Moeller Apr. 6, 1943 2,325,126 Giesler July 27, 1943 2,428,523 Marshall Oct. 7, 1947 2,481,962 Whitfield Sept. 13, 1949 2,544,670 Grange Mar. 13, 1951 2,544,671 Grange Mar. 13, 1951 2,569,097 Grange Sept. 25, 1951 2,604,415 Whitfield July 22, 1952 FOREIGN PATENTS 7 10,459 Great Britain Apr. 22, 1893 166,506 Switzerland Mar. 1, 1934 425,596 Great Britain Mar. 29, 1934 57,756 Norway Mar. 22, 1937 454,951 Canada Mar. 8, 1949 660,404 Great Britain Nov. 7, 1951 OTHER REFERENCES Welding Encyclopedia, Eleventh Edition, 1943, published by The Welding Engineer Publishing Co., Chicago, Ill., page 618. 

1. THE PROCESS OF FORMING A STRUCTURE HIGHLY RESISTANT TO OXIDIZATION AT ELEVATED TEMPERATURE WHICH CONSISTS IN COPPER BRAZING TOGETHER A PLURALITY OF FERROUS METAL PARTS TO PROVIDE AN ASSEMBLY OF FERROUS METAL PARTS HAVING EXPOSED COPPER BRAZING MATERIAL BETWEEN THE FERROUS METAL PARTS, PREHEATING THE ASSEMBLY TO A TEMPERATURE AT LEAST AS HIGH AS THE MELTING POINT OF ALUMINUM IN A SALT BATH CONSISTING SUBSTANTIALLY AS FOLLOWS; 